This invention relates to optical waveguide transmission systems and, more particularly, to superluminescent light-emitting diodes that can deliver light to an optical waveguide or fiber with maximum efficiency.
Although semiconductor lasers have achieved outstanding success in serving as optical sources in fiber waveguide systems, there are still many problems to be solved with respect to these devices. Specifically, many of the lasers do not have lifetimes that approach the other components in the fiber waveguide systems and their manufacturing yield is still not as high as desired. Some theories on the lack of an adequate laser lifetime have suggested that the presence of optical peaks within the cavity of the laser may, in fact, contribute to the rapid degradation of the device. Light-emitting diodes, on the other hand, produce incoherent radiation and, therefore, do not have standing waves of optical energy within their active region and for this reason may exhibit much longer lifetimes than their laser cousins. Light-emitting diodes manufactured to date, however, do not permit coupling the amount of optical power into a fiber that is required for systems applications and, therefore, the use of superluminescent light-emitting diodes (SLD) has been suggested. SLDs are superior to ordinary light-emitting diodes (LEDs) in that they have larger modulation bandwidth and smaller spectral width. In this type of light-emitting diode, the active region is pumped to a level such that gain is experienced by the light beams that are generated within the active region, but the device is purposely made to have no optical resonator to prevent laser oscillation.
One type of superluminescent light-emitting diode is disclosed in the article entitled "A Stripe-Geometry Double-Heterostructure Amplified-Spontaneous-Emission (Superluminescent) Diode" by T. P. Lee et al, IEEE Journal of Quantum Electronics, Vol. QE-9, No. 8, August 1973, pp. 820-828. In the SLD disclosed in this article a stripe-geometry current confining area is established by the contact on a double-heterostructure device and the contact is purposely made shorter than the overall length of the semiconductor chip in order to ensure that the radiation is absorbed in the unpumped region not covered by the contact. In this way, laser operation is prevented. The article by T. P. Lee et al also suggests making the "geometric" numerical aperture of the active region substantially equal to the numerical aperture of the fiber into which the light is to be coupled.